Method and Device for Reading Data

ABSTRACT

The present invention relates to a method of reading data from a record carrier, the method comprising the steps of: obtaining a readout signal by radiating the record carrier in a first mode and a second mode of operation of a light source, computing a quality factor of the readout signal for the first mode and the second mode of operation of the light source, determining a best mode of operation of the light source based on the computed quality factors and selecting the determined best mode of operation of the light source for reading data from the record carrier. This is useful for high-speed Blu-ray disc applications.

The invention relates to a method of reading data from a record carrier.

The invention also relates to a device for reading data from a recordcarrier.

The invention also relates to a software program for reading data from arecord carrier.

Data information is recorded on a record carrier, e.g. as a series ofpits and lands representing binary data and forming a track. Thisinformation recorded on the record carrier, e.g. CD, DVD, Blu-ray discis retrieved from a readout signal. A light/laser spot remains locked tothe track for scanning. For optically scanning the rotating disc, anoptical disc drive comprises a light beam generator, e.g. a laser diode,an objective lens for focusing the light/laser beam onto a spot on thedisc and an optical detector for receiving the light reflected from thedisc and for generating an electric detector signal. The reflected lightspot is incident on the photodetector. The corresponding readout signalis derived from the photodetector. This readout signal takes amaximum/minimum value. The value taken by the readout signal depends onthe amount of noise that reduces the level of the readout signal. Duringreadout of an optical disc, the current supplied to the light/lasersource is usually modulated with a sinusoid at a frequency of 200-500MHz in order to suppress noise. This laser modulation method is used inCD-R/RW, DVD and Blu-ray players/recorders. An embodiment of an opticaldisc device is disclosed in JP63276720 wherein the laser current ismodulated during reading of the optical disc. The modulation is appliedin order to reduce the laser noise by varying a high-frequencysuperimposing current in accordance with the reproducing power of theoptical disc. Thus, a modulation factor is optimized and the laser noiseis reduced. There is a drawback of using laser modulation in a Blu-rayplayer/recorder as disclosed in JP63276720. The drawback of using lasermodulation is illustrated in FIG. 1. FIG. 1 shows a plot of light outputof a laser source when laser modulation is applied. The peaks of thelight power are much larger than the average value, and in order toguarantee readout without degradation of the written data, the peaksshould not exceed a certain limit. Therefore, the Blu-ray standardspecifies that the maximum averaged readout power should not be largerthan 0.3 mW in the case of laser modulation and not larger than 0.4 mWwhen no laser modulation is applied (1× speed). This power differencecorresponds to a difference of 2.5 dB in favor of no modulation. Theother drawback of applying laser modulation is that noise of the laserdriver is larger when it has to generate a high frequency-modulatedlaser current. This further drawback of using laser modulation isillustrated in FIG. 2. FIG. 2 shows a plot of spectral density of thenoise current in nA/√Hz of the output of a typical laser driver used inmost optical pick-up units available on the market. In FIG. 2, the floorlevel is the noise level of the used spectrum analyzer (laser isswitched off), No HFM is the noise level of the laser light when nolaser modulation is applied, 20 mApp is the noise level of the laserlight when the laser is modulated with a HF current of 20 mApeak-to-peak, and 30 mApp is the noise level of the laser light when thelaser current is modulated with a HF current of 30 mA peak-to-peak. Itcan be observed from FIG. 2 that there is a large increase of noise whenlaser modulation is applied. It depends on other noise sources in theread channel how much overall decrease of signal-to-noise ratio thisimplies. Hence, if laser modulation is applied to suppress laserfeedback noise at low readout powers, it does not have a significanteffect on the signal-to-noise ratio of the readout signal. There aremany factors that influence the amount of feedback noise in a laser,such as:

a) distance of laser to disc,b) the amount of returning light in %,c) the type of laser,d) the used average laser power and the polarization properties of thelight path.

It turns out that the feedback noise of Blue lasers in a Blu-ray discplayer/recorder is low, which is mainly due to the very low percentageof light power that returns into the laser facet. At low readout powers,the amount of laser noise is less and hence the solution of using lasermodulation does not always guarantee a positive effect on thesignal-to-noise ratio of the readout signal, in particular for Blu-raymedia.

It is therefore an object of the invention to provide a method ofreading data from a record carrier that optimizes the signal-to-noiseratio of the readout signal.

It is a second object of the invention to provide a device for readingdata from a record carrier that optimizes the signal-to-noise ratio ofthe readout signal.

It is a third object of the invention to provide a software program forreading data from a record carrier that optimizes the signal-to-noiseratio of the readout signal.

The first object of the invention is realized by providing a method ofreading data from a record carrier, the method comprising the steps of:

obtaining a readout signal by radiating the record carrier in a firstmode and a second mode of operation of a light source,

computing a quality factor of the readout signal for the first mode andthe second mode of operation of the light source,

determining a best mode of operation of the light source based on thecomputed quality factors,

selecting the determined best mode of operation of the light source forreading data from the record carrier.

The first object of the invention is achieved by selecting a mode ofoperation of the light source that results in a higher signal-to-noiseratio of the readout signal, thereby improving the readout performance.

In an embodiment of the method, the first mode of operation of the lightsource comprises modulating the current supplied to the light source byswitching on the light modulator, and the second mode of operationcomprises not modulating the current supplied to the light source, i.e.switching off the light modulator during reading of the record carrier.Modulation is not applied if its application has a contrary effect, i.e.if it has the effect of decreasing the signal-to-noise ratio of thereadout signal rather than increasing it. During readout, the two modesof operation, namely modulating the current supplied to the lightsource, i.e. modulator on, and not modulating the current supplied tothe light source, i.e. modulator off, are compared. The effect of eachmode of operation on the readout signal is compared for a particularrecord carrier at a particular speed. The mode that gives a higher levelof signal-to-noise ratio of the readout signal is selected and used forreading data from the record carrier. This selection of the mode ofoperating the light source prevents the application of modulationwithout any justification.

In a further embodiment of the method, the method further comprisesobtaining the readout signal during initialization, using maximumallowed read power for the first mode of operation of the light source,i.e. modulator on, and using maximum allowed read power for the secondmode of operation of the light source, i.e. modulator off. It isadvantageous to carry out the measurement at start-up, becauseafterwards the optical drive must read the user data under the bestpossible conditions. Hence, the measurement is done before any user datais read. By using a higher read power, the signal energy will be higherand the signal-to-noise ratio will consequently be higher.

In a still further embodiment of the method, the quality factor iscomputed on the basis of the jitter range of radial tilt. Very often,the optical drive measures this jitter range during start-up in order tofind an optimum setting of the radial tilt actuator. The optical drivethus simply needs to measure each point twice, corresponding to the tworead conditions, which hardly requires extra time. As the jitter curveis rather narrow in shape, the difference between the two conditions canbe determined accurately.

In a still further embodiment of the method, the quality factor iscomputed on the basis of the jitter range of focus offset. Very often,the optical drive measures this jitter range during start-up in order tofind an optimum setting of the focus actuator. The optical drive thussimply needs to measure each point twice, corresponding to the two readconditions, which hardly requires extra time. As the focus jitter curveis rather narrow in shape, the difference between the two conditions canbe determined accurately.

In a still further embodiment, the method finds a mode of operation ofthe laser source that gives a wider quality factor window to determinethe best mode of operation of the light source for reading data from therecord carrier. The quality factor window is the jitter range of radialtilt or the jitter range of focus offset. The idea here is to use avariable that gives a pronounced jitter window when varying it along asuitable range. Selecting a wider quality factor, i.e. a wider jitterrange makes the system more robust against e.g. tilted discs, bad mediaquality and misaligned optics.

In the methods of the embodiment, the record carrier is a Blu-ray disc,because the signal-to-noise ratio is very critical for Blu-ray discreadout and more so at higher speeds.

The second object of the invention is realized by providing a device forreading data from a record carrier, the device comprising:

readout means arranged to obtain a readout signal by radiating therecord carrier in a first mode and a second mode of operation of a lightsource,

computing means arranged to compute a quality factor of the readoutsignal for the first mode and the second mode of operation of the lightsource,

determining means arranged to determine a best mode of operation of thelight source based on the computed quality factors,

selecting means arranged to select the determined best mode of operationof the light source for reading data from the record carrier.

In an embodiment of the device, the device is a Blu-ray discplayer/reader.

The third object of the invention is realized by providing a softwareprogram comprising an executable code for performing the method ofreading data as described in the previous paragraphs.

These and other objects of the invention will be further elucidated anddescribed, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 shows a plot of light output of a laser source when lasermodulation is applied,

FIG. 2 shows a plot of spectral density of the noise current in nA/√Hzof the output of a typical laser driver,

FIG. 3 is a flow chart of a method of reading data from a record carrierin accordance with an example of an embodiment of the invention,

FIG. 4 shows details of a further embodiment for computing the qualityfactor based on the jitter range of radial tilt,

FIG. 5 is a block diagram that depicts one example of a player device,in particular a Blu-ray disc player to which the present invention isapplied.

FIGS. 1 and 2 have been described in detail in the opening paragraph andwill therefore not be described again.

FIG. 3 is a flow chart of a method of reading data from a record carrierin accordance with an example of an embodiment of the invention. Forensuring a robust reading of data stored on a record carrier, e.g. CD,DVD Blu-ray disc, it is required to generate a readout signal that has agood quality. Data recovery becomes difficult if the input readoutsignal has a low amplitude and is very noisy. Hence, a method of readingdata from the record carrier, in particular a Blu-ray optical disc isproposed, wherein the readout signal has a higher signal-to-noise ratio,thereby improving the readout performance. In step 302, the lightsource, e.g. Blue laser is operated in a first mode of operation,wherein the current supplied to the light source, e.g. a Blue laserdiode for reading data is modulated, i.e. the laser modulator isswitched on. The laser modulator here refers to a high-frequencymodulator to suppress laser feedback noise. For Blu-ray disc readout,the frequency of this modulator is typically 400 MHz, which is muchhigher than the frequency of the written data. In step 304, the readoutsignal Readout_(with mod) is obtained. In step 306, a quality factorQF_(with mod) is computed from the readout signal Readout_(with mod). Instep 308, the light source, e.g. Blue laser diode is operated in asecond mode of operation, wherein the laser current supplied to thelaser source, e.g. Blue laser diode for reading data is not modulated,i.e. the laser modulator is switched off. In step 310, the readoutsignal Readout_(no mod) is obtained. In step 312, a quality factorQF_(no mod) is computed from the readout signal Readout_(no mod). Instep 314, QF_(with mod) is compared with QF_(no mod) and a best mode ofoperating the light source is determined. In step 316, the determinedbest mode of operating the light source is selected for reading datafrom the record carrier.

In an embodiment of the method according to the invention, the readoutsignals Readout_(with mod) and Readout_(no mod) are measured at initialstart-up of the optical drive. The measurement of the readout signal isdone by using a maximum allowed read power for the first mode ofoperation of the light source, i.e. laser modulator on, and using amaximum allowed read power for the second mode of operation of the lightsource, i.e. laser modulator off, e.g. the Blu-ray disc standardspecifies that readout power should be 0.3 mW with laser modulation and0.4 mW without laser modulation. Hence, using the respective readoutpowers, the readout signal is obtained. Before attempting to read datafrom the record carrier, e.g. Blu-ray disc, the reading system must beinitialized. This means that the value of certain system parameters mustbe initially determined so that the laser beam can be properly focusedonto the spot and the tracks can be properly followed. It isadvantageous to carry out the measurement at start-up, becauseafterwards the optical drive must read the user data under the bestpossible conditions. Hence, the measurement is done before any user datais read. By using a higher read power, the signal energy will be higherand consequently the signal-to-noise ratio will be higher.

In a still further embodiment of the method according to the presentinvention, the quality factor for each mode of operation of the lightsource, e.g. Blue laser diode is computed on the basis of the jitterrange of radial tilt. Principally, a record carrier, e.g. a Blu-ray discshould be kept in a flat disc shape when it is set in a disc motor sothat an optical pick-up unit can keep its optical axis perpendicular tothe recording surface of the disc during reproducing operation. Forscanning the recorded tracks, the optical pick-up unit moves in a radialdirection in alignment with the radius of the disc. However, the discset in the disc motor is not flat, mainly due to the manufacturingprocess. The disc curves in both radial and circumferential directions.Therefore, the optical pick-up unit cannot scan the recording trackswith its optical axis perpendicular to the recording surface of thedisc. Furthermore, the angle varies in accordance with the position ofthe optical pick-up unit with respect to the disc. The angle formedbetween the optical axis and the recording surface in a radial directionis defined as radial tilt angle. Due to e.g. time errors in thehigh-frequency signal used for extracting data, a certain amount ofjitter is always present when reading out an optical disc. The tiltangle between the disc and the objective lens is a result of two majorcontributors, namely the disc (contribution by manufacturing tolerancesand environmental changes) and the drive (contribution by objective lensactuator, turntable motor adjustment, axis adjustment, disc speed, etc).The resulting angular deviations lead to a distortion of the opticalreadout spot on the disc. This distorted readout spot directly resultsin a distorted readout signal and thus in timing errors, i.e. jitter.Generally, the jitter increases at a greater rate as the radial tiltbecomes larger. The computation of the quality factor based on thejitter range of radial tilt involves the following steps, and a possibleresult of this procedure is shown in FIG. 4.

1. Applying laser modulation by switching on the laser modulator andsetting the read power at the maximum allowed value when using lasermodulation, e.g. P=0.33 mW for a Blu-ray disc.

2. Dividing the range of radial tilt into N suitable points, e.g. N=15points between −1 deg and +1 deg.

3. Measuring the jitter of a piece of data for each N tilt value. Usingthe same data for each tilt value.

4. Applying a suitable curve fitting (e.g. a parabola) on the foundjitter values.

5. Calculating the width of the window W₁ of the jitter curve at somepredefined limit value Jmax.

6. Switching off the laser modulation and setting the read power at themaximum allowed value when using no laser modulation, e.g. P=0.4 mW fora Blu-ray disc.

7. Repeating steps 3 to 5 and finding the width of the window W₂ of thejitter curve.

8. Determining which window is wider, W₁ or W₂.

9. Selecting the mode of operation of the laser source that gives thewider window to read the user data.

FIG. 4 shows that not applying laser modulation gives a wider window W₂than applying laser modulation. This is not always the case. It is to benoted that this result may be different under different conditions, andthat is why a measurement is done at start-up. Furthermore, selecting awider window gives the advantage of having a system that is more robustagainst e.g. tilted discs, bad media quality and misaligned optics. Inessence, the two modes of operation of the laser source, namely:

Mode 1: Low laser power and modulator on

Mode 2: High laser power and modulator off are compared. The mode thatgives a wider margin is selected for reading data.

Very often, the optical drive measures this jitter range during start-upin order to find an optimum setting of the radial tilt actuator. Theoptical drive thus simply needs to measure each point twice,corresponding to the two read conditions, which hardly requires extratime. As the jitter curve is rather narrow in shape, the differencebetween the two conditions can be determined accurately. Although HD-DVDdoes not distinguish between modulated and non-modulated read power, theabove procedure can still be used to find the best mode of operation forreading a HD-DVD.

In a still further embodiment of the method according to the invention,the quality factor for each mode of operation of the light source, e.g.a Blue laser diode is computed on the basis of the jitter range of focusoffset. The procedure for computing the quality factor is similar asdescribed for radial tilt in the previous paragraph. The optical drivecan choose to use either radial tilt or focus offset or both or someother variable. The idea here is to use a variable that gives apronounced jitter window when varying it along a suitable range.

FIG. 5 is a block diagram that depicts one example of a player device,in particular a Blu-ray disc player 500 to which the present inventioncan be applied. Referring to FIG. 5, a record carrier 502, e.g. aBlu-ray optical disc is rotationally driven by a spindle motor 504. Anoptical pick-up unit 506 radiates the Blu-ray disc with laser lightemitted by the Blue laser light source 508, e.g. a Blue laser diode. TheBlue laser light source 508 is operated in two different modes duringreading of data from the Blu-ray disc 502. In a first mode of operation,the current supplied to the Blue laser source 508 is modulated, i.e.laser modulator 508 a is switched on. The frequency of the lasermodulator 508 a is typically 400 MHz, which is much higher than thefrequency of the written data. In a second mode of operation, thecurrent supplied to the Blue laser source 508 is not modulated, i.e.laser modulator 508 a is switched off. The optical pick-up unit 506 thenreceives light reflected on the Blu-ray disc 502 to generate a signal inaccordance with the intensity of the received light. A photodetector 510disposed within the optical pick-up unit 506, e.g. a photo diodereceives the beam of light reflected on the Blu-ray disc 502 and thenconverts the light into electric signals, which in turn are output asread signals Rd_sig. The read signals Rd_sig are applied to a digitalsignal-processing unit (not shown) of the microcontroller 512. Thedigital signal-processing unit processes the received read signals andthe processed signal is accordingly output to a video external outputterminal V_(term) and/or to an audio external output terminal A_(term).The optical pick-up unit 506 is also provided with a tracking actuator(not shown) and a focusing actuator (not shown). The tracking actuatorchanges the direction of an objective lens (not shown) to displace aread point of the laser beam spot in the radial direction of the Blu-raydisc 502. The focusing actuator controls the focus position of theaforementioned laser beam spot. In the above configuration, the opticalpick-up unit 506 reads information recorded on the record carrier andoutputs a read signal Rd_sig. A unit 514 extracts a tracking errorsignal and a focusing error signal from the read signal Rd_sig. Thetracking error signal and the focusing error signal are supplied to adriver 516 which uses the tracking error signal and the focusing errorsignal to control the tracking actuator (not shown), the focusingactuator (not shown), the slide motor (not shown) and the spindle motor504. The Blu-ray disc player 500 further comprises readout means 518arranged to obtain the readout signal Rd_sig for a first mode ofoperation of the blue laser source 508, i.e. laser modulator 508 a on,and a second mode of operation of the Blue laser source 508, i.e. lasermodulator 508 a off. The Blu-ray disc player 500 further comprisescomputing means 520 to compute a quality factor of the readout signalRd_sig for the first mode of operation and the second mode of operationof the Blue laser source 508. The Blu-ray disc player 500 furthercomprises determining means 522 to determine a best mode of operation ofthe Blue laser source 508 based on the computed quality factors. TheBlu-ray disc player further comprises means 524 arranged to select thedetermined best mode of operation of the Blue laser source 508 forreading data from the record carrier 502. The construction shown in FIG.5 only illustrates portions related to the general operation of aBlu-ray player device. The description and detailed explanation ofservocircuits for controlling the optical pick-up unit, the spindlemotor, the slide motor, the digital signal-processing unit and the likeare omitted, because they are constructed in a similar manner as inconventional systems.

In embodiments of the invention, the quality factor is computed on thebasis of the jitter range of radial tilt or focus offset. Alternatively,the proposed method of the invention can be implemented by measuringwith and without modulation, the bit error rate window or block errorrate window of focus offset or radial tilt. Then the mode of operationof the light source that gives a wider window of bit error rate or blockerror rate can be selected for reading data from the record carrier.

In embodiments of the invention, the disclosed method of reading datafrom a record carrier may be implemented as a software programcomprising an executable code to be executed by a device such as aBlu-ray player/reader for reading/playing data. When executed, thesoftware program causes the device to perform the steps of the claimedmethods for reading data from the Blu-ray disc by selecting the bestmode of operating the laser source.

Although the invention has been mainly explained with reference toembodiments using optical discs, it is also suitable for other recordcarriers such as rectangular optical cards or any other type ofinformation carrier that uses a light source and a light spot forreading data. A person skilled in the art can implement the describedembodiments of the method of reading data from a record carrier insoftware or in both hardware and software. It will, however, be evidentthat various modifications and changes may be made without departingfrom the broader scope of the invention, as set forth in the appendedclaims. Use of the verb “comprise” and its conjugations does not excludethe presence of elements or steps other than those stated in a claim orin the description. Use of the indefinite article “a” or “an” precedingan element or step does not exclude the presence of a plurality of suchelements or steps. The Figures and description are to be regarded forillustrative purposes only and may not be used to limit the invention.

In summary, the invention provides a method of reading data from arecord carrier, the method comprising the steps of:

obtaining a readout signal by radiating the record carrier in a firstmode and a second mode of operation of a light source,

computing a quality factor of the readout signal for the first mode andthe second mode of operation of the light source,

determining a best mode of operation of the light source based on thecomputed quality factors,

selecting the determined best mode of operation of the light source forreading data from the record carrier.

This is useful for high-speed Blu-ray disc applications.

1. A method of reading data from a record carrier (502), the methodcomprising the steps of: obtaining a readout signal by radiating therecord carrier in a first mode and a second mode of operation of a lightsource, computing a quality factor of the readout signal for the firstmode and the second mode of operation of the light source, determining abest mode of operation of the light source based on the computed qualityfactors, selecting the determined best mode of operation of the lightsource for reading data from the record carrier.
 2. The method asclaimed in claim 1, wherein, in the first mode of operation of the lightsource, a light modulator (508 a) supplying current to the light source(508) is switched on during reading of the record carrier and, in thesecond mode of operation of the light source, the light modulator (508a) supplying current to the light source (508) is switched off duringreading of the record carrier.
 3. The method as claimed in claim 2, themethod further comprising obtaining the readout signal duringinitialization using maximum allowed read power for the first mode ofoperation and using maximum allowed read power for the second mode ofoperation of the light source.
 4. The method as claimed in claim 1,wherein the quality factor is computed on the basis of the jitter rangeof radial tilt.
 5. The method as claimed in claim 1, wherein the qualityfactor is computed on the basis of the jitter range of focus offset. 6.The method as claimed in claim 4, wherein the best mode of operation ofthe light source is a mode of operation that gives a wider qualityfactor window.
 7. The method as claimed in claim 1, wherein the recordcarrier is a Blu-ray disc.
 8. A device for reading data from a recordcarrier, the device comprising: readout means (518) arranged to obtain areadout signal by radiating the record carrier in a first mode and asecond mode of operation of a light source, computing means (520)arranged to compute a quality factor of the readout signal for the firstmode and the second mode of operation of the light source, determiningmeans (522) arranged to determine a best mode of operation of the lightsource based on the computed quality factors, selecting means (524)arranged to select the determined best mode of operation of the lightsource for reading data from the record carrier.
 9. The device asclaimed in claim 8, wherein the device is a Blu-ray disc player/reader.10. A software program comprising an executable code for performing allthe steps of claim 1.